just to be clear: when you run each interface pair member as primary for a comparable period of time, the IO rates are not the same by a factor up to 1.8? Are the interface machines on the same subnet? Is the network latency the same for each machine on the link to the PI server and to the OPC Server? Any packet loss on the links from the slower machine (use PATHPING to see)?

Have you looked at the UniInt IO Rate for the interface instead? Under UniInt->Health Points. If not, let's start there to see if it is code or something else. There are also tags for IO Rate for each scan class in there.